How this calculator works
Add the running watts of everything that will be on at the same time. Motors (fridges, pumps, AC) briefly draw extra power to start; since they rarely start at the same instant, the calculator adds only the single largest extra starting load to the running total. A safety margin keeps the generator from running at its limit.
Running watts = Σ (Running watts × Quantity)
Starting watts = Running watts + largest (Start − Run) of the items on
Recommended = Starting watts × (1 + Margin%/100), rounded up to 0.5 kW
Assumptions and tips
- Wattages are typical values for planning. Use the nameplate or manual of your own appliances where you can.
- Generator running (continuous) rating should also cover the running total with margin.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Refrigerator — 700 W run / 2,200 W start | 1 |
| Chest freezer — 500 W run / 1,500 W start | 0 |
| Sump pump (1/2 hp) — 1,050 W run / 2,150 W start | 1 |
| Furnace blower (1/2 hp) — 800 W run / 2,350 W start | 0 |
| Window AC (10,000 BTU) — 1,200 W run / 3,600 W start | 0 |
| Washing machine — 1,150 W run / 2,250 W start | 0 |
| Microwave — 1,000 W run | 0 |
| Coffee maker — 1,000 W run | 0 |
| Space heater — 1,500 W run | 0 |
| LED lights (per 10 bulbs) — 100 W run | 1 |
| TV + internet router — 250 W run | 1 |
| Laptops / phone chargers — 100 W run | 1 |
| Other equipment: running watts | 0 W |
| Other equipment: extra starting watts | 0 W |
| Safety margin | 20 % |
| Result | Value |
|---|---|
| Recommended generator size | 4.5 kW (starting/peak rating) |
| Total running watts | 2,200 W |
| Peak starting watts | 3,700 W |
| Running watts with margin | 2,640 W |
Step by step:
- Running watts = 2,200 W
- Largest extra starting load = 1,500 W (fridge)
- Starting watts = 2,200 + 1,500 = 3,700 W
- With 20% margin = 4,440 W
Common mistakes to avoid
- Adding every appliance’s starting watts together — motors rarely start at the same moment.
- Back-feeding a house through an outlet; it can electrocute utility workers.
Frequently asked questions
What size generator do I need to run a refrigerator?
A typical fridge uses about 700 W running and 2,200 W to start, so even a small 3,000 W generator can handle it with lights and chargers.
What is the difference between running and starting watts?
Running watts power equipment continuously; starting (surge) watts are the brief extra power a motor needs to start.